The ability to switch optical signals is fundamental to further advances in many fields of optics. For example, in optical communications networks it is necessary to efficiently and rapidly switch signals propagating via waveguides, such as optical fibers. Specifically, what is required are optical 1×2 and 2×2 switches as well as more general N×M switches capable of single or gang operation.
The optical switching techniques taught by the prior art can generally be grouped into all-optical and mechanical approaches. All-optical approaches promise extremely rapid switching speeds but have a number of significant drawbacks, including low efficiency, low reliability and high cost. These limitations eliminate all-optical switches from applications in fiberoptic networks where high levels of reliability, longevity, switching precision and low cost are key.
A number of mechanical approaches to optical switching rely on external optical elements such as reflectors, prisms, gratings and shutters. In switches using such optical elements the signals are outcoupled from a given output fiber and are then incident on the external optical element. The optical element is mechanically adjusted to reflect, refract, diffract or otherwise pass the light from the output fiber to the intended input fiber to thus perform its switching function. In most of these switches the fibers are fixed and the optical elements are positioned on movable stages activated by solenoids, piezoelectric members or other mechanical actuators. The need to displace optical elements to achieve switching limits the speed and accuracy of these types of optical switches rendering them inadequate for fiber optic networks.
A number of mechanical optical switches move the fiber itself to effectuate switching. A number of these solutions involve bending the fibers to couple the optical signals between the chosen fibers. For example, U.S. Pat. No. 4,303,302 issued to Ramsey et al. teaches a piezoelectric optical switch which has a piezoelectric element with an optical fiber attached to it. A second optical fiber is placed in general proximity to the first optical fiber so that the fibers can be aligned by the piezoelectric element upon application of an appropriate voltage causing the piezoelectric element to bend and the mechanically coupled fibers to move into their predetermined switching position. Similarly, U.S. Pat. No. 4,657,339 issued to Fick teaches a fiber optic switch which bends the fiber with the aid of a piezoelectric element to achieve switching. Still other teachings, including U.S. Pat. No. 4,512,036, U.S. Pat. No. 4,543,663 and U.S. Pat. No. 4,651,343 all issued to Laor, teach bending the fiber with the aid of a bender assembly along a circumferential direction to couple signals between fibers arranged circularly around the fiber being bent. Patent Application US2002/60025108 teaches bending the fiber with the aid of a push-rod. Yet another teaching provided by Yutaka Ohmori et al., “Optical Fiber Switch Driven by PZT Bimorph”, Applied Optics, Vol. 17, No. 22, 15 Nov., 1978, pp. 3531-3532 teaches to bend a fiber from a central position between two butted optical fibers by using a bimorph constructed with two sheets of PZT ceramics.
One drawback to mechanical switches, which bend fibers, is maintaining fiber planarity. Specifically, the bending of a fiber is not a very controllable process and it is difficult to keep the fiber in one plane as it is being bent. Therefore, switches relying on bending fibers suffer from alignment problems, thus rendering them inadequate for high speed and high accuracy optical networks.
Another drawback of prior art mechanical switches that utilize piezoelectric elements are difficulties in maintaining positioning accuracy and positioning repeatability. The bending of the piezoelectric element is dependent on the input voltage and effected by the resilience of the involved moving parts. Aging and varying operational conditions like voltage or temperature fluctuations limit positioning accuracy and repeatability during the devices lifetime.
Mechanical switches, which move the fiber without bending it, are taught in the prior art, such as in U.S. Pat. No. 5,864,643 issued to Pan. Pan teaches a miniature 1×N electromechanical optical switch and variable attenuator which has an array of end sections of output optical fibers, an end section of an input optical fiber and an actuator to effect a relative movement of the input optical fiber end section with respect to the output optical fiber end sections to form an optical path between the input optical fiber and a selected output optical fiber. Switching is performed by moving the input optical fiber in the plane of the array. Pan's switch uses an alignment controller for performing two alignment steps including a coarse alignment and then a fine alignment based on a feedback signal dependent on the alignment.
U.S. Patent Application US2001/0048785 to Steinberg teaches the use of a passive latching interface in the configuration of a roller element traveling in a groove for selectively coupling one or more fibers of a first array to one or more inputs of fibers of a second array. The groove in which the roller element travels has detents to facilitate more accurate registration or alignment between fibers of the first and second arrays. Steinberg's passive latching interface provides additional positioning accuracy on the expense of increased switching forces and eventual wear of the contacting parts of the latching interface.
Still other approaches employing a parallel translation of fibers by using a ball bearing and a sliding frame are described in U.S. Patent Applications US2002/0003921 and US2002/0025107. These solutions introduce well-known mechanical elements to provide a latching in predetermined switching positions. The prior art latching interfaces are passive and introduce additional parts that are prone to wear.
Therefore, there exists a need for a simple and robust optical switch providing for rapid switching, mechanical stability, accurate fiber alignment as well as longevity. The switch architecture should be adaptable to 1×2, 2×2 and N×M switches capable of single or gang operation.